Cv

Coefficient of Variation

How to calculate the homogeneity of a dry mix

Do you have a question, a remark ? Please contact the author at powder.process@protonmail.com

Section summary
How to calculate Cv
Confidence intervals and how to assess a mix

Powder Mixing (main page)
# Types of mixers
# Mixing mechanisms
# Mixing time
# Mixing speed
# Mixing volume
# Mixing power
#Mixer loading # Discharge time
# Froude Number
# Coefficient of Variation
# Homogeneity
# Tracer
# Segregation / demixing
# Tip speed
# Sampling
# ATEX speed
# Mixer Bearing seals
# Mixer discharge valve

Calculation of CV homogeneity

After sampling, the samples taken are analysed regarding the presence of the tracer. Then results are recorded and statistics can be peformed.

The mixture, regarding the tracer, will be defined according to the mean concentration of the tracer and to the standard deviation of the tracer concentration. Mean and standard deviation will be used to calculate the relative standard deviation of the mix, that will be the value assimilated to "homogeneity".

Relative Standard Deviation

Equation 1 : Relative Standard Deviation of the mixture

S is the samples standard deviation, it is not the actual standard deviation since only an estimation can be given from the samples. μ is the arithemical average of the samples concentration, thus as well calculated from the samples.

This value is often expressed in % and called also the coefficient of variation

Calculation of CV

Equation 2 : Coefficient of variation of the mixture

The value obtained is then compared to the specification.

WARNING : the CV that will be obtained has actually several components, and some of these components need to be calculated in order to estimate the actual homogeneity variance.

The sample variance is calculated thanks to :

S2=Smix2+Sanalytical2+(Ssampling2)

The variability due to sampling is very difficult to determine, thus, in practice, it will be neglected. However, for this assumption to be true it is critical to sample the mix following the methods explained above, and preferably on the free flowing powder.

The variability due to analysis can either be known, if experiments have been done before, or can be determined for the particular homogeneity validation by doubling the measurement on the same sample.

Smix can then be calculated. Then CVmix(%)

Is the mix ok ? Confidence intervals

WARNING : Once CVmix(%) calculated, it is not sufficient to compare it to the specification. Indeed, the variance calculated is NOT a true variance but is estimated on the sampling. This estimation must be taken into account by calculating a confidence interval, generally at 95%, which corresponds to 2 sigma on each side of the mean.

The specification can then be compared to the confidence interval :
- If the CVspec > upper border of the confidence interval : the mixing is successful, the mix is significantly better than the specification
- If CVspec is in the confidence interval : the mixing may be successful, but it is also possible that the actual CVmix is > CVspec
- If CVspec is < lower border of the confidence interval : the mixing homogeneity achieved is not good enough for the application.

For the 2 last cases, case by case discussion will be necessary within the factory team :
- Accept the mix
- Reject the mix and look for root causes (mixer speed, filling rate of mixer...)




Do you want to know more ?
www.powderprocess.net is covering all aspects of solids and powder processing and handling
The pages below will probably interest you !
PNEUMATIC TRANSPORT HANDBOOK
Answers ALL your questions on pneumatic transport
QUALITY of MIX / MEASURING DEGREE OF MIXING
All you need to know about measuring the degree of mixing (Coefficient of Variation) of a dry mix
SEGREGATION OF POWDERS / DEMIXING
All on the physical phenomena causing segregation of particles and how to avoid them
GRINDING and MILLING
All on main size reduction processes, key equipement, design consideration..etc..
POWDER Properties
All on powder key physical characteristics, among others : flowability, density...etc..
EQUIPMENT HANDBOOK
All equipment found in bulk solids handling industries, explained, with design tips...
You can find much more through the menus at the top of the page !